Communication systems are known to comprise a plurality of communication units, such as vehicle-mounted mobile or hand-held portable radios, and a limited number of communication resources that are transceived by at least one base station. For example, trunked communication systems operate by sharing the limited number of communication resources, which may be pairs of radio frequencies, among the plurality of communication units over a large coverage area. Typically, the large coverage area includes several sites (geographic areas in which communications coverage is provided by a base station or set of base stations), each site covering only a portion of the large coverage area. By dedicating one channel within each site of the trunked communication system as a control channel, a central controller is able to dynamically allocate the remaining channels, often called voice channels, so that multiple intra-group or dispatcher-to-group communications may take place concurrently. Such systems are useful to entities accommodating a large number of communication units, such as delivery or utility services, which require a large, but limited, coverage area.
It is often the case that sites within a trunked communication system have varying user densities. The user density of a site can be defined as the average number of communication units operating within the site at any given time. Thus, high-density sites typically require a larger number of communication resources and base stations to meet user needs. Likewise, in a low-density site, i.e., a site typically having a relatively small number of operating communication units, there is a lesser need for multiple communication resources and base stations. Indeed, it is not uncommon to find two-channel sites having only a control channel and a single voice channel. Low-density sites are generally not cost-effective in that they relatively infrequently--as compared to high-density sites--utilize their dedicated infrastructure and spectrum resources. The inefficiency of low-density sites often proves to be a financial burden to trunked communication system users and a competitive disadvantage to trunked communication system manufacturers.
A simple solution to this situation is to simply exclude low-density sites from a trunked communication system. That is, any site not having a sufficiently high user density is eliminated at the cost of reduced overall coverage area. This is an inadequate solution for systems requiring a large number of low-density sites. For example, public safety and/or utility organizations providing services to predominantly rural areas typically do not have high user densities. Nevertheless, the services that such organizations provide are often crucial, and provision of these services can be hampered by reduced overall coverage area.
Another solution is the use of a control channel capable of converting to voice channel operation when necessary. This is particularly useful in low-density sites since a single channel can be used to provide coverage, thus reducing the need for costly infrastructure and spectrum resources. A potential disadvantage of this solution is that once the control channel has been assigned as a voice channel, services typically provided by the control channel, such as emergency requests and call queuing requests, are no longer available. This presents a potentially critical, and often unacceptable, reduction in services. Therefore, a need exists for a method that provides communication units with access to a trunked communication system, particularly in low-density sites, without the need for increased infrastructure and spectrum resources.